sys: add support for NetBSD getvfsstat

NetBSD replaced the getfsstat interface with getvfsstat in NetBSD 3.0.

Add a test for it.

With help from Benny Siegert and Ian Lance Taylor.

Change-Id: I115ae48c287cc32181006e9e8f7c15851e7e36c0
Reviewed-on: https://go-review.googlesource.com/c/sys/+/550476
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Benny Siegert <bsiegert@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>